This invention relates to apparatus for continuously producing photovoltaic devices by depositing successive layers of semiconductor material onto a substrate as that substrate travels through operatively connected, dedicated deposition chambers, the composition of the semiconductor layers being dependent upon, inter alia, the particular process gas mixtures introduced into each of the deposition chambers. The deposition chambers are connected by a relatively narrow gas gate passageway through which the substrate material passes and which is adapted to isolate the process gases introduced into the first chamber from the process gases introduced into the adjacent deposition chamber. Despite the relatively small size of the gas gate passageways, a percentage of gases introduced into one chamber still back diffuses into the adjacent chamber, thereby contaminating the layer deposited in said adjacent chamber. In an effort to reduce the diffusion of process gases into adjacent chambers, deposition apparatus constructed by the assignee of the instant application have incorporated shields which at least partially surround the cathode region and which cooperate with introduction and evacuation conduits to inhibit the free flow of process gases from the cathode region. The process gases introduced into the cathode region are therefore directed to flow across the substrate for disassociation into plasma and subsequent deposition onto that substrate. However, the process gases, so introduced have been found to form flow patterns as they are deposited as semiconductor layers onto the surface of the substrate, thereby reducing the efficiency of photovoltaic devices produced therefrom.
In order to better understand the causes of flow line formation, it is helpful to refer to FIG. 4 in which the process gas supply conduit is depicted by the reference numeral 36. The supply conduit 36 is an elongated member including a plurality of spaced apertures 39 through which a plurality of streams of process gas mixtures 43a-43h enter the deposition chamber and are directed to the plasma region thereof. However, the streams of the process gas mixtures 43a-43h, retain their individual velocities and chemical compositions for a distance "d" as they travel from the supply conduit 36 toward the plasma region. As they reach the distance "d", adjacent streams of process gas mixture 43a-43h begin to interdiffuse to form a single, uniform, homogeneous stream of the process gas mixtures. Based upon the foregoing explanation, the flow pattern formation may now be understood as being due to the fact that (1) certain zones of the plasma region (defined as that region between the substrate and cathode in which an electrodynamic field is developed) may have streams of process gas mixtures flowing therethrough at a relatively slow velocity, while adjacent zones of the plasma region may have streams of process gas mixtures move quickly therethrough, and (2) the discrete streams of process gas mixtures may also include slightly different percentages of gaseous constituents, thereby forming mixtures of nonuniform and nonhomogeneous chemical composition. If the discrete streams of process gas mixtures from the plurality of spaced apertures are not provided with a sufficiently lengthy path of travel (the distance "d") prior to contacting the electrodynamic field developed in the plasma region, the nonhomogeneity and nonuniformity thereof will result in the localized deposition of nonhomogeneous, nonuniform semiconductor material upon surface of the substrate. And, since the rate of deposition of the semiconductor material onto the surface of the substrate is proportional, all other parameters being kept constant, to (1) the length of time which the process gases are subjected to the electrodynamic field and (2) the chemical composition of the process gas mixtures exposed to the electrodynamic field, flow patterns may be formed on the surface of the substrate in at least two ways. First, any localized zones, defined by the adjacent slowly and more rapidly moving streams of process gas mixtures, will necessarily mean that adjacent streams of process gas mixtures remain in the plasma region and are subjected to the electrodynamic field for varying lengths of time. The result is that the more slowly moving streams of process gas mixtures are deposited at a different rate than the streams of process gas mixtures which move more rapidly through the plasma region. Also, the slower moving process gas mixtures, being exposed to the field for a greater length of time than the faster moving mixtures, deposit compositionally different films onto the substrate surface. The differences in the length of time which process gas mixtures spend subjected to the electrodynamic field within the plasma region thereby defines the first manner in which the aforementioned flow patterns are formed.
The second manner in which flow patterns may be formed by semiconductor material depositing onto the surface of the substrate relates to the fact that the chemical composition of the process gas mixtures introduced into the plasma region from each of the plurality of spaced, discrete apertures vary slightly. Since (1) the chemical compositions of the process gas mixtures vary, and (2) different materials are deposited onto the substrate at different rates of deposition, the compositionally varying process gas mixtures deposit compositionally varying semiconductor material onto the surface of the substrate, thereby forming flow patterns indicative of the localized deposition of nonuniform, nonhomogeneous semiconductor material.
Further, in the deposition apparatus constructed by the assignee of this application, although each deposition chamber included a shield to direct the process gases through the plasma region to an evacuation port for withdrawal, the web of substrate was adapted to pass thereabove. Therefore, only the central section of the surface of the substrate was available for depositing semiconductor material thereonto. Accordingly, the prior art shield arrangements failed to make maximum use of the substrate surface area available for the production of semiconductor devices.
The present invention operates to substantially (1) reduce the formation of flow patterns on the layered substrate surface caused by either (a) the different rates of flow of the process gas mixtures traveling through the plasma region of a deposition chamber or (b) nonuniform, nonhomogeneous process gas mixtures entering that plasma region, and (2) expose the entire surface of the substrate traveling through the plasma region of a deposition chamber for depositing thereonto semiconductor material.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type devices which are, in operation, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques which possess (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such a technique is fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors, which issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, which issued in the names of Stanford R. Ovshinsky and Masatsugu Izu on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498, issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially, the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by smaller band gap materials to absorb the light passed through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltages from each cell may be added, thereby making the greatest use of the light energy passing through the semiconductor device.
It is of obvious commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon, which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications and patents: U.S. Pat. No. 4,400,409 issued Aug. 23, 1983 for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; U.S. Pat. No. 4,410,558, issued Oct. 18, 1983, for Continuous Amorphous Solar Cell Production System; U.S. Pat. No. 4,438,723, issued Mar. 27, 1984 for Multiple Chamber Deposition And Isolation System And Method; and U.S. Pat. No. 4,492,181, issued Jan. 8, 1985, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material.
In making a solar cell of p-i-n type configuration, the first chamber is preferably dedicated for depositing a p-type amorphous silicon alloy, the second chamber is preferably dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is preferably dedicated for depositing an n-type amorphous silicon alloy. Since each deposited alloy, and especially the intrinsic alloy must be of high purity, the deposition environment in the intrinsic deposition chamber is isolated from the doping constituents within the other chambers to prevent the back diffusion of doping constituents into the intrinsic chamber. In the previously mentioned patent applications, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by gas gates through which unidirectional gas flow is established and through which an inert gas may be "swept" about the web of substrate material.
While the foregoing discussion dealt with the production of semiconductor devices from a continuously moving web of substrate material, the process gas introduction, confinement and evacuation system of the present invention is equally adaptable for use with batch processing apparatus because it is equally likely that zones of localized process gas velocity will be formed from nonuniform, nonhomogeneous process gas mixtures introduced into those batch processing chambers which employ process gas introduction systems structurally similar to those of continuous process apparatus.
Regardless of whether the glow discharge deposition chamber is adapted for the continuous or batch production of semiconductor devices, the movement of adjacent streams of process gases from the apertured introduction manifold though the plasma region, as well as the homogeneity and uniformity of those adjacent streams of process gases introduced into the plasma region from the spaced apertures of the introduction manifold, as discussed hereinabove, can cause the nonuniform, nonhomogeneous deposition of semiconductor material onto the surface of the substrate, thereby forming flow patterns. Further, in said prior chambers, the edges of the substrate were covered by the flanges of the cathode shield, thereby preventing semiconductor material from being deposited thereonto.
It is therefore one object of the present invention to provide apparatus which will substantially reduce the formation of flow patterns of semiconductor layers deposited onto the substrate surface, said patterns caused by (1) zones of slower and more rapidly moving streams of process gas mixtures passing through the plasma region of the deposition chamber, and (2) the flow of nonuniform, nonhomogeneous process gas mixtures through the plasma region of the deposition chamber.
It is another object of the present invention to increase the length of the opposed, horizontally extending flanges of the cathode shield so that the substrate can be positioned below the flange for exposing the entire transverse surface of said substrate for the deposition of semiconductor material thereonto.
These and other objects and advantages of the present invention will become clear from the drawings, the claims and the detailed description of the invention which follow.